1. Field of the Invention
The present invention relates generally to a Visible Light Communication (VLC) method and system, and more particularly, to a VLC method and system using a time-sharing scheme.
2. Description of the Related Art
As the luminous efficiency of Light Emitting Diodes (LEDs) has improved and their price has fallen, the use of LEDs has become popular not only in the special lighting markets for portable devices, displays, automobiles, traffic lights, billboards, or the like, but also in the general lighting markets for fluorescent lamps, incandescent electric lamps, or the like. In addition, studies of VLC using visible light LEDs are being made in many enterprises, as the interest in an optical wireless technology that complements a Radio Frequency (RF) technology increases due to exhaustion of RF band frequencies, possible conflict between several wireless communication technologies, increased demand for communication security, advent of super-high speed ubiquitous communication environments in a 4th Generation (4G) wireless technology, etc.
VLC that communicates information using visible light is secure and its bandwidth is broad and can be freely used without restriction. In addition, since VLC makes it possible for users to see the places where the light arrives, and the direction in which the light propagates, the users may accurately perceive the coverage where the light can be received. Therefore, VLC is advantageous in that it is reliable in terms of security and can be driven with low power. Due to these advantages, VLC is applicable even in hospitals and airplanes where the use of RF is generally banned. Also, VLC can provide value-added services. A description of the VLC system will be given below with reference to drawings.
FIG. 1 illustrates a configuration of a general system using VLC (hereinafter referred to as a “VLC system”). The general VLC system includes a light source(s) 10 and a VLC terminal 20. The light source 10 is composed of a plurality of LEDs or Laser Diodes (LDs) to serve as a lighting apparatus and perform data transmission/reception using visible light. The VLC terminal 20 includes a visible light transmission/reception module to perform data exchange with the light source 10. The VLC terminal 20 may include mobile terminals such as a mobile phone and a Personal Digital Assistant (PDA), and fixed terminals such as a desktop computer. In addition, VLC may be combined with communication systems using other wire/wireless communication media, to ensure more efficient use.
A general structure of the VLC terminal 20 is illustrated in FIG. 2. Referring to FIG. 2, the VLC terminal 20 includes a transmission/reception controller 21, an encoder 22, an LED driver 23, an LED 24, a photodiode 25, a detector/receiver 26, and a decoder 27.
The transmission/reception controller 21, which controls the overall operation of the VLC terminal 20, processes data for VLC data transmission/reception by controlling the encoder 22 and the decoder 27. The encoder 22 encodes transmission data input from the transmission/reception controller 21 and outputs the encoded transmission data to the LED driver 23. The LED driver 23 optically modulates the data input from the encoder 22, and drives the LED 24 so that the transmission data can be transferred to an external device.
The photodiode 25 senses an optical signal provided from the external device, converts the sensed optical signal into an electrical signal, and outputs the electrical signal to the detector/receiver 26. The detector/receiver 26 demodulates the electrical signal input from the photodiode 25 into data that is based on optical wireless communication, and outputs the demodulated data to the decoder 27. The decoder 27 decodes input data and outputs the decoded data to the transmission/reception controller 21, and the transmission/reception controller 21 properly processes the received data input from the decoder 27.
Meanwhile, when VLC services are to be provided in a broad space, a plurality of light sources 10 are installed in the space depending on service coverage of each light source 10. For example, as illustrated in FIG. 3, a plurality of light sources 10 can be installed on the ceiling of one room in a lattice form at specified intervals, taking into account a unit service area in which each light source 10 can provide VLC services.
When a plurality of light sources 10 are installed at specified intervals as illustrated in FIG. 3, substantial service areas of the light sources 10 may appear in three possible cases as shown in FIGS. 4A to 4C, in which service areas of two light sources 31 and 33 are shown by way of example. In this situation, the service areas of the light sources 31 and 33 may partially overlap each other as shown in FIG. 4A, may contact with each other at their edges as shown in FIG. 4B, or may be spaced apart from each other as shown in FIG. 4C, providing a non-service area 43 between the service areas. The case of FIG. 4B, which is an ideal state, is substantially impossible. Generally, the service areas of the multiple light sources may include an overlapping area where they overlap in part as shown in FIG. 4A, or a non-service area may exist between two service areas as shown in FIG. 4C.
However, when the light source 31 and the light source 33 provide different types of services, collisions may occur between data transmitted by the two services in the overlapping area, making it difficult to provide normal services. Further, normal VLC communication is impossible in the non-service area. In order for adjacent light sources to normally provide VLC services, they provide the same services to all users, placing a limit on the capability of VLC.